1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an optical pickup device capable of reproducing discs of two types having different densities by double beam focuses of different sizes.
2. Description of the Prior Art
Information reproducing/recording systems for using optical discs such as laser discs or compact discs have been developed in recent years. Such optical discs have a variety of information, e.g., generally, a digital audio disc is associated with the reproducing of musical sounds and a digital video disc is with the reproducing of images. These discs are recorded with information at a proper recording density in accordance with the purpose of usage. For example, the digital audio disc for reproducing musical sounds is recorded with smaller density, and the digital video disc for reproducing images is with larger density, e.g., as many as four times. The information stored into the inside of the above-mentioned discs is readable/writable by means of optical pickup devices, and can reproduce the discs of different kinds having different densities by means of optical pickup devices of different kinds.
A construction and operation of a general optical pickup device is disclosed in U.S. Pat. No. 4,767,921 or 4,868,377.
The conventionally general optical pickup device will be described in detail with reference to FIG. 1 below.
FIG. 1 is a schematic view showing the conventionally general optical pickup device. In FIG. 1, a reference numeral 11 denotes a light source for generating laser beam, which is a laser diode 11. The laser beam emitted from laser diode 11 is diffracted while transmitting through a diffraction grating 12. The diffracted beam proceeds toward a beam splitter 13.
Here, beam splitter 13 is provided in such a manner that two right-angled prisms are installed to oppose to each other against respective inclination planes of 45.degree., and a coating layer 13a is formed along the contacting portion of the prisms, thereby transmitting some of the incident beam and reflecting the other incident beam to be perpendicular to the incident beam while securing the property of travelling straight of the incident beam.
The diffracted beam is reflected toward optical disc 15 by means of beam splitter 13. The beam reflected from beam splitter 13 focuses onto a recording plane 15a of an optical disc 15 while passing through an objective lens 14 installed in front of a recording medium such as optical disc 15. A positional accuracy, i.e., focusing error and tracking error, of the pickup device with respect to optical disc 15 is detected from an image of the beam received into a photodetector 16, and therefore, the focusing and tracking are controllable. Also, the information is read out on the basis of the amount of the reflected light determined by pits in recording plane 15a of optical disc 15.
Meantime, if the recording capacity of the disc is to be four times like the digital video disc over the digital audio disc, the width of pits in the disc is consequently decreased by 1/2. For this reason, a focused spot size onto the digital video disc should be half of that onto the digital audio disc with the consequence of necessarily employing different optical pickup devices in reproducing the digital video disc and digital audio disc by means of the conventional optical pickup devices. That is, in order to accurately read out data respectively recorded on the digital audio disc and digital video disc, it is required to separately apply an optical pickup device for digital audio disc of which beam spot size is approximately 1.6 .mu.m when focusing onto the digital audio disc and an optical pickup device for digital video disc of which beam spot size is approximately 0.8 .mu.m when focusing onto the digital video disc.